When (meth)acrylic acid and its esters are prepared by conventional reaction processes, these products are generally present in a mixed product gas exiting the reactors of such processes. Typically, mixed product gases containing (meth)acrylic acid are cooled and contacted with an aqueous stream in an absorption tower, thereby providing a crude product stream which is then purified, often by dehydration in a distillation step, to provide a more concentrated and pure product solution. Mixed product gases containing esters of (meth)acrylic acid are generally non-aqueous streams, often with alcohols as the solvent-carrier, which are also subjected to purification by distillation, which also results in formation of a more concentrated and pure product solution.
Unfortunately, under the high temperature conditions typical of purification by distillation, (meth)acrylic acid and its esters have a strong tendency to polymerize, creating unwanted polymer solids in the process equipment. It is important to prevent such unwanted polymerization of (meth)acrylic acid and its esters because process equipment may become blocked and clogged with the polymer solids which tends to interfere, sometimes dangerously, with ongoing operation of the purification process. In addition, loss of product yield often results from the polymerization of a portion of the product.
One established method for inhibiting polymerization of (meth)acrylic acid and its esters involves addition of one or more substances, which inhibit such polymerization, to the process streams and process equipment during separation and purification operations. More particularly, polymerization inhibitors that comprise methylene blue (tetramethylthionine chloride), along with one or more other compounds, such as hydroquinone (HQ) and phenothiazine (PTZ), have been found to provide good polymerization inhibiting effects for (meth)acrylic acid and its esters.
For example, such a method was described in U.S. Pat. No. 4,021,310, which teaches the addition of a combination of three substances to a distillation column during separation and purification of a crude aqueous stream containing acrylic acid or its esters. More particularly, the distillation was carried out in the presence of a three-component inhibitor system comprising: (A) at least one compound selected from the group consisting of hydroquinone, hydroquinonemonomethyl ether, cresols, phenols, t-butyl catechol, diphenylamine, phenothiazines and methylene blue; (B) at least one compound selected from the group consisting of copper dimethyldithiocarbamate, copper diethyldithiocarbamate copper dibutyldithiocarbamate and copper salicylate; and (C) molecular oxygen. According to the disclosure of this patent, this method achieved a synergistic polymerization inhibiting effect when the aforesaid compounds were simultaneously present in the distillation of aqueous streams containing acrylic acid and its esters.
However, it has also been learned that the halide (i.e., chloride) anions in methylene blue compound tend to corrode separation and purification process equipment constructed of low nickel content stainless steel, including but not limited to those grades containing less than about 35 weight percent nickel, such as grades 304, 316, and 317L. Although metal alloys and coatings (e.g., glass) that are resistant to chloride corrosion are commercially available, their costs are significantly higher than low nickel content stainless steel and are, therefore, less attractive substitutes for constructing commercial process apparatus. Furthermore, unless specifically removed with the attendant additional costs of such removal, the chloride of methylene blue will continue to cause negative effects in process apparatus downstream of the separation and purification portion, through corrosion and other unwanted interactions with these less expensive materials of construction.
Thus, removal and/or substitution of a substantial portion of the chloride (i.e., halide) anions in methylene blue (a thiazine dye-based compound) would provide reduced halide-content thiazine-based compounds that are suitable for use as polymerization inhibitors having reduced corrosion and other unwanted effects. Similarly, since methylene blue is a member of the class of compounds known as azine dyes, which includes thiazine-, oxazine-, and diazine dye-based compounds, removal and/or substitution of a substantial portion of the halide anions in azine-based compounds would provide various reduced halide-content azine dye-based compounds suitable for use as polymerization inhibitors in processes for separation and purification of (meth)acrylic acid and its esters.
Suitable reduced halide-content azine-based compounds may be available in readily usable form from various commercial sources. For example, 5-amino-4-(diethylamino)benzo[a]-phenoxazinium hydrogen sulfate (Nile Blue A) (which has an oxazine dye-based cation) or 3,7-Diaminophenothizain-5-ium acetate (thionine acetate) (which has a thiazine dye-based cation) are commercially available and suitable inhibitor compounds which would be expected to inhibit polymerization of (meth)acrylic acid and its esters. Since these compounds are of reduced halide-content, they would also be expected to provide polymerization inhibiting effects while reducing corrosion of process equipment compared to compounds containing halides, such as chloride. Furthermore, such inhibitor compounds having azine dye-based cations are suitable for use alone or with other compounds, such as those mentioned above, which also have polymerization inhibiting activity.
It is also desirable to be able to convert halide-containing azine dye-based compounds, such as, for example, tetramethylthionine chloride(methylene blue), into reduced halide-content azine-based compounds suitable for use as polymerization inhibitors, either alone or with other inhibitor compounds. An ion exchange process, wherein the halide anions of such azine dye-based compounds are exchanged for another less-corrosive anion species, would achieve the aforesaid desired conversion. Generally, ion exchange processes exchange one ion in solution for another ion that is bound to a substrate.
For example, U.S. Pat. No. 3,641,016 discloses methods for preparing thionine derivatives using ion exchange processes conducted in solution, which do not involve ion exchange resins and, therefore, require additional drying, filtration, and/or washing steps to remove the solvent and unwanted inorganic alkaline salts from the final thionine derivative products. In particular, Example 13 of U.S. Pat. No. 3,641,016 describes the formation of the hydroxide form of thionine compounds by treatment with a strong base, i.e., potassium hydroxide or sodium hydroxide, in alcoholic solution. The solution is dried by evaporation and the alkaline salt is removed by washing the solids with a small quantity of water, leaving the purified, dry thionine hydroxide product. Example 14 of U.S. Pat. No. 3,641,016 describes the formation of thionine salts having organic and mineral acids (such as acetates, fumarates, halogens, and sulfates) by an ion exchange process wherein the hydroxy groups of the thionine hydroxide compounds, in either aqueous or alcoholic solution, are replaced by the preferred organic or mineral acid anions. Where aqueous solutions are used, the newly formed thionine compound is separated from the solution by filtration and removal of additional organic and inorganic alkaline salts is achieved by washing with a small quantity of water. Where the solution is alcoholic, after the hydroxy groups of the thionine hydroxide compounds are replaced by the preferred organic or mineral acid anions, the solution must be dried, as by evaporation, to collect the solids, which are then washed with a small quantity of water to remove the inorganic alkaline salts. In addition, Example 15 of U.S. Pat. No. 3,641,016 discusses the preparation of thionine derivatives having anions derived from carboxylic acids by an ion exchange in aqueous solution between the thionine chloride, bromide or perchlorate compound and the sodium, potassium or ammonium salt of the desired carboxylic acid. The filtration and washing steps are also required in this process to separate and purify the resulting carboxylic acid form of the thionine compound.
It is also noted that U.S. Pat. No. 6,458,989 discloses the use of sulfonic salts as inhibitors in the production of (meth)acrylic acid and its esters, however, only sulfonic salts of phenothiazine are illustrated in the examples. Furthermore, while phenothianzine is a compound belonging to the class of compounds known as azines, phenothiazine is not an azine dye-based compound, with which the present invention is concerned. See Smith, M. and March, J., March's Advanced Organic Chemistry Reactions, Mechanisms, and Structure, 5th Ed., 2001, John Wiley & Sons, Inc., New York, p. 310; and R. Raue in Ullmans Encyclopedia of Industrial Chemistry, 5th Ed., 1985, VCH Deerfield Beach Fla., Vol. A3, p. 213.
Various suitable resin substrates are known in the industry and are conventionally made from polymer material or zeolites. While ion exchange processes, and the ion exchange resins used in such processes, are generally known and developed for application in various fields, there are a number of difficulties encountered with such processes as particularly applied to the conversion of azine dye-based compounds.
For example, U.S. Pat. No. 4,813,974 describes an ion exchange process which reduces the halide (e.g., chloride) content of cationic or basic dyes, such as tetramethylthionine chloride, but the disclosed process is designed to remove only the stoichiometric excess halide anions. Although the use of strongly basic ion exchange resins would be a clear way to remove halide ions associated with the dye molecules, U.S. Pat. No. 4,813,974 warns against the use of strongly basic ion exchange resins due to the resulting high pH which tends, in turn, to render the resulting salt product highly unstable. U.S. Pat. No. 4,813,974 explains how to avoid removing too much halide by matching the total capacity of the ion exchange resin loaded into the reactor with the excess halide content of a single batch of dye being treated, which necessarily results in leaving the stoichiometric halide in the dye. Thus, while U.S. Pat. No. 4,813,974 suggests that all or any portion of the halide anions directly associated with the cation portion of the starting compound may be removed and subsequently replaced with substitute anions, that patent warns that this may be achieved only if the temporary exposure to the resulting higher pH does not adversely affect the cation portion and does not provide any information or guidance as to how to avoid or mitigate such adverse affect. Thus, U.S. Pat. No. 4,813,974 does not explain how to successfully address the difficulties and obstacles of which it warns.
In addition, it has been known in the art that methylene blue is considered to be incompatible with basic conditions (i.e., high pH environments). See The Merck Index, 11th Addition, 1989, Merck & Co., Rahway N.J., p. 954. Furthermore within the dye industry, azine-based dyes, of which methylene blue is a member, are classified as basic dyes (i.e., the chromophore is cationic). See Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 3, 4th addition, 1992, John Wiley & Sons, New York p 814. As indicated in Ullmann's Encyclopedia of Industrial Chemistry, 5th completely revised edition, Volume A9 VCH, Deerfield Beach, Fla., p. 107, members of this class of dyes are generally considered to be sensitive to basic conditions because sodium hydroxide has been reported to change or decolorize solutions of basic dyes (i.e., the cation portion is destroyed). Thus, persons of ordinary skill are dissuaded, by the general knowledge and information in the art, from using ion exchange resins with a high pH environment to synthesize derivatives of azine dye-based compounds, including derivatives having reduced halide content.
Notwithstanding the foregoing, the present invention provides a method of inhibiting polymerization of (meth)acrylic acid and its esters, for example during separation and purification processes, as well as transport and storage, using a polymerization inhibitor which comprises at least one reduced halide-content azine dye-based compound.
The present invention also provides a process for preparing reduced halide-content azine dye-based compounds using ion exchange resin technology, which addresses and overcomes the difficulties of known ion exchange processes discussed above.